The demand for wireless communications can be said to be inversely related to the cost of providing portable radio transceivers. Hence it is preferred that such transceivers be provided substantially in an integrated circuit form to minimize cost.
A radio transceiver has two functions, one being that of broadcasting an information signal and the other being that of receiving an information signal. The two functions may occur coincidentally at different frequencies, as in full duplex operation, or the two functions may be mutually exclusive such that the function of receiving is interrupted when the broadcasting function occurs, as in simplex operation. In simplex operation, the broadcast signal can be at the same frequency as the signals being received. The transceiver is switched between the broadcast and receive functions to effect information exchange with another party either directly or via some intervening communications network. In one example the transceiver periodically switches between the broadcast and receive functions to effect exchanges of packetized data at a rates which appear to provide full duplex operation for a user.
Sophisticated transceivers based on superheterodyne principles have evolved over the years such that the weakest of recognizable signals can be usefully received and signals can be broadcast with very precise frequency and bandwidth control. One example is found in U.S. Pat. No. 5,423,076 issued Jun. 6, 1995 to Larry L. Westergren et al, titled “Superheterodyne Transceiver with Bilateral First Mixer and Dual Phase Locked Loop Frequency Control”. The receive function is performed by circuitry which includes triple down conversion, requiring local oscillators, mixers and attendant bandpass filters for a receive frequency band as well as each of three successively lower intermediate frequencies.
In U.S. Pat. No. 6,006,081 titled “Communications Receivers” and issued Dec. 21, 1999, Paul A. Moore discloses an integrated zero IF receiver which receives an input signal and down converts it to substantially zero intermediate frequency quadrature related signals (I,Q). In one example the received signal is supplied to a pair of mixers. A local oscillator signal is supplied to one of the mixers directly and a phase shifted version of the local oscillator signal is supplied to the other of the mixers. The products of mixing from the one mixer are applied to a first lowpass filter which passes an in-phase difference signal (I) a demodulator. The products of mixing from the other mixer 16 are applied to a second low-pass filter 24 which passes a quadrature related difference signal (Q) to the demodulator. The demodulator recovers the original modulation. Tuning to any desired frequency is accomplished by varying the local oscillator signal.
The examples disclosed by Moore and Westergren et al are generally representative of the field of radio apparatus in that each requires a plurality of filters which can be difficult if not impossible to provide in integrated circuit form. Provision of such in discrete form with the required interconnections adds significantly to the expense of manufacture of small portable radio devices such as receivers and transceivers.